usb-host: fix zero-length packets

usb-host optimizes away zero-length packets by not entering the
processing loop at all.  Which isn't correct, we should submit a
zero-length urb to the host devicein that case.  This patch makes
sure we run the processing loop at least once.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/host-linux.c

@@ -887,8 +887,8 @@ static int usb_host_handle_data(USBDevice *dev, USBPacket *p)
     prem = 0;
     pbuf = NULL;
     rem = p->iov.size;
-    while (rem) {
-        if (prem == 0) {
+    do {
+        if (prem == 0 && rem > 0) {
             assert(v < p->iov.niov);
             prem = p->iov.iov[v].iov_len;
             pbuf = p->iov.iov[v].iov_base;
@@ -938,7 +938,7 @@ static int usb_host_handle_data(USBDevice *dev, USBPacket *p)
                 return USB_RET_STALL;
             }
         }
-    }
+    } while (rem > 0);
 
     return USB_RET_ASYNC;
 }